The majority of microbial sensors which measure the concentration of a matter, measure the concentration of a matter in various solutions by utilizing a change of the microbial respiration. When microorganisms are put in a sample solution containing various matters and they metabolize those matters, their respiration is activated intensely, which will lead to reduced oxygen concentration in the environment surrounding them. Such changes in oxygen concentration are measured by oxygen electrodes, and this is the method by which a microbial sensor can measure the concentration of a substance.
For example, BOD (biochemical oxygen demand) is an important factor to be considered when the quality of water in rivers and sewage from plants must be controlled, and has been taken internationally as an indicator of the organic pollution of a water. The water pollutants derived from organic compounds are degraded through oxidation by aerobic microorganisms and consumed in the end, and in correspondence with the reduced concentration of those organic matters, dissolved oxygen is also consumed. The measurement of how much oxygen is consumed represents the pollution of a given water. In other words, BOD represents the concentration of organic compounds in terms of the oxygen consumed. A method for measuring BOD is regulated by the Japanese Industrial Standard, JIS K 0102. However, this method requires an intricate operation and it takes five days to measure BOD by this method, and hence it comes to be replaced the JIS K 0102 method by a method which allows a quick, simple and on-line measurement of BOD (JIS K 3602.sup.-1990). As a BOD sensor for this kind of method, a BOD sensor which depends on the use of a combination of microbial film and oxygen electrodes (Japanese Laid-Open Patent Application No. 54-47699, etc.), has been provided and has been utilized for the measurement of BOD in industrial waste water or the like.
Besides BOD sensors described above, the sensor utilizing microbial activity includes a developed ethanol sensor which combines a membrane upon which are immobilized microorganisms such as Trichosporon brassicae which selectively consume ethanol and thus breathe vigorously in the presence of ethanol, and oxygen electrodes which measure the reduction in oxygen concentration in a solution which occurs as a result of activated respiration.
However, in the sensor described above which determines the concentration of matters in a solution by measuring the reduction in dissolved oxygen with oxygen electrodes, there is a problem that it is difficult to determine exactly the concentration of matters of a solution where dissolved oxygen remains at a low level. In addition, as the oxygen electrode must contain an electrolyte and the like within its structure, it is forced to have a certain size. Accordingly, the conventional microbial sensors which are often installed into a fermenter or the like to determine the concentration of various matters and BOD sensors described above are all large or medium in size and expensive.